Should the system already be up and running with plants "installed"?
In answer to this question, don't forget the Scuds need something to eat to survive. The plants don't have to be installed, but there will have at least be something in the grow bed or bio-filter for them to eat. If you have algae in the water or bio-filter that will help.

I saw you mentioned they live in the grow medium, I still haven't planted what I intend to grow yet.
They don't care if you have planted anything or not

Still haven't had more then one day of running the system. Would it be safe to put in now or wait until the system has had time to grow?
Your system will need to mature a bit, at least a couple weeks with fish in the mix. Do you have any fish as of yet?